1. Field of the invention
The present invention relates to a spectroscopic method for measuring the concentrations of sugars such as glucose, saccharose, fructose and the like and, more particularly, to a method and apparatus for measuring sugar concentrations in foods, fruits and other agricultural products, and the like and in body fluids of man, animals, and other creatures by a non-invasive or non-destructive and easily repeatable technique, that is, without taking the sugar medium as sample for measurement out of the object of examination.
2. Related art
A non-invasive and easily repeatable measuring method as mentioned above is specially useful in that the method dispenses with the step of taking a sample, for example the body fluid, out of the object of examination in the measurement procedure. In this method, however, it is unsatisfactory that the regions amenable to the measurement should be limited to the outermost layer of the object of examination. For example, in a fruit as an object of such examination, the pericarp which constitutes the outermost layer is different from the sarcocarp in the interior of the fruit with respect to the structure of the tissue, and the composition, the related distribution, and the like of the chemical component. The determination of the sugar concentration in the sarcocarp necessitates relevant measurement at deeper positions than the outermost layer.
In U.S. Pat. No. 4,655,225 is described a prior art technique whereby sugars in human serum, especially glucose therein, can be quantitatively determined non-invasively. This technique, known as "incident angle modulation method", is a spectroscopic method wherein, in principle, a sample is irradiated with light beams from the outside and the light beams diffused and reflected from within the sample are spectroscopically analyzed. In this method the angle of incidence which light waves makes with a sample is changed. When this angle is small, the light beams penetrate deep underneath the outermost layer, and when the angle is large, the depth to which the light beam penetrates decreases. Therefore, by changing the depth to which a light beam penetrates underneath the outermost layer so as to find the respective spectral signals from the different depths, information from a deeper point, that is, the sugar concentration, can be discretely determined on the basis of the differential signal therebetween.
However, the above-mentioned prior art technique when practiced raises a problem in that the mechanism for modulating the angle of incidence is complicated on the one hand and the change in the incidence angle of light beam impairs the reproducibility under the influence of a resultant change in the reflection characteristic at the surface of the outer layer.
In said prior art method the light beam used for measurement are of wavelengths in the near infrared region, which are 2,270.+-.15 nm, 2,100.+-.15 nm, 1,765.+-.15 mn, and 1,575.+-.15 nm, and the reference wavelengths used are in the range of 1,000 to 2,700 nm. In order to enhance the accuracy of the measurement, however, shorter wavelengths in the near infrared region should be used for the measurement. The reason is that the liquid in a living body, especially in a fruit, agricultural product, or the like, consists of water in such a large proportion that the optical penetration depth of light beams to water, which assumes a larger degree on the side of shorter wavelengths, is of importance in the practice.
Wavelengths in the intermediate infrared range, especially those in the range of 7,500 to 15,000 nm called "fingerprint region", are effective in spectral analyses and have long since been used for identification and determination of organic compounds. Since, however, the water existing as a background exhibits such a high rate of absorption for said wavelength region that it is generally considered impractical to determine a specific component combined with water by the fingerprint region. Although light beams in visible region have a good optical penetrability in water, there exists no spectrum of a characteristic absorption bands for sugar in the visible light region.